Modern telecommunications networks are capable of offering various types of services at ever greater bandwidths and speeds. Examples of services include both circuit-switched services (e.g., voice-based telephony sessions) and packet-switched services (e.g., electronic mail, web browsing, electronic gaming, packet-switched voice communications, and so forth).
There are two types of networks, circuit-switched networks and packet-switched networks. In a circuit-switched network, a circuit is allocated to each communications session (such as a voice-based call session) between endpoints. This circuit is occupied for the duration of the communications session between the endpoints. For example, in a circuit-switched network that employs time-division multiplexing, an allocated circuit may include a time slot from a group of multiple time slots. The allocated circuit is not released until the communications session is terminated, which means the circuit is unavailable for other sessions.
In contrast, the resources of a packet-switched network can be shared by multiple communications sessions, with bandwidth on the packet-switched network provided on an as-needed basis. In other words, even though a communications session may be active between two endpoints, network resources are not allocated to the session if there are no packets to send, which means that the network resources are available for other sessions. In packet-switched communications, packets carrying address information are transmitted, with the address information in the packets used to route the packets from a source to a destination.
Although there has been a high proliferation of packet-switched networks, with the Internet being a well known example, circuit-switched networks still have many applications. For example, in addition to circuit-switched networks used in public switched telephone networks (PSTNs), circuit-switched links are also used to interconnect nodes of a mobile or wireless communications network. A mobile communications network is made up a plurality of cells, with each cell containing a radio communications center in which a mobile station can establish a call with another endpoint. Each cell typically includes a base transceiver system (BTS), which includes a radio frequency (RF) transceiver to communicate radio signals with mobile stations within a cell. The BTSs are coupled by links to a base station controller (BSC). Multiple BSCs can in turn be coupled to a mobile switching center (MSC). In alternative arrangements, base stations at corresponding cell sites are coupled directly to the MSC, instead of through BSCs.
Examples of circuit-switched links include T-carrier links (e.g., T1 or DS1, T2 or DS2, T3 or DS3, etc.). In Europe and Japan, variations of the T-carrier concept include E-carrier links (e.g., E1, etc.) and J-carrier links (e.g., J1, etc.). A T-carrier link employs time division multiplexing (TDM) to derive multiple channels from a physical wire-based circuit. In some cases, instead of electrical wires, T-carrier links are carried over an optical transport to further enhance bandwidth between the various nodes of a mobile communications network.
Circuit-switched networks also have numerous other applications, such as to link user terminals by local exchange devices, e.g., key telephone systems, private branch exchange (PBXs), automatic call distributors (ACDs), and so forth.
In many telecommunication networks, synchronous operation is a technique used to achieve high performance. In some cases, precise synchronization of nodes within a distributed network is employed to achieve high performance. For example, in some mobile communications networks, global positioning system (GPS) receivers are provided in each BTS to achieve synchronization among the BTSs. Because there usually are a large number of BTSs in a mobile communications network, providing a GPS receiver in each BTS can be quite costly to implement.